Architecture and morphodynamics of subcritical sediment waves in an ancient channel-lobe transition zone

In modern systems, submarine channel-lobe transition zones show a well-documented assemblage of depositional and erosional bedforms. In contrast, the stratigraphic record of channel-lobe transition zones is poorly constrained, because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings. Several locations from an exhumed fine-grained base of slope system (Unit B, Laingsburg depocentre, Karoo Basin, South Africa) show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions. The regional stratigraphic context, lack of a basal confining surface, wave-like morphology in dip section, size and facies characteristics support an interpretation of subcritical sediment waves within a channel-lobe transition zone setting. Some sediment waves show steep (10 to 25 degrees) unevenly spaced (10 to 100m) internal truncation surfaces that are dominantly upstream-facing, which suggests significant spatio-temporal fluctuations in flow character. Their architecture indicates that individual sediment wave beds accrete upstream, in which each swell initiates individually. Lateral switching of the flow core is invoked to explain the sporadic upstream-facing truncation surfaces, and complex facies distributions vertically within each sediment wave. Variations in bedform character are related to the axial to marginal positions within a channel-lobe transition zone. The depositional processes documented do not correspond with known bedform development under supercritical conditions. The proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions, and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture.